1. Field of the Invention
The present invention relates to a soft ferrite material for manufacturing devices for shielding electromagnetic frequencies such as wire coiling inductors and chip components including chip inductors and chip beads. The present invention also relates to a method for manufacturing an inductor by using the ferrite material. More specifically, the present invention relates to a Ni--Cu--Zn soft ferrite material and a method for manufacturing a wire coiling type or chip type inductor using the ferrite material, whereto low temperature sintering is possible, the variations of the electromagnetic properties are low as against an external stress, and the electromagnetic properties are superior.
2. Description of the Prior Art
Recent progress in the manufacte of electronic and communication apparatus has led to the production of miniaturized thin and easily installable electronic components, resulting in a new industrial structure which gave rise to new problems such as environmental pollution and communication interferences, which could previously be disregarded. Due the proliferation of wireless communication apparatuses and the multi-tech environment, the electromagnetic environment has been aggravated. Accordingly, the nations of the world came to reinforce the electromagnetic impediment regulations (FCC, CISPR, VDE, MIL).
Therefore, electromagnetic wave impediment removing devices (EMI/EMC) are in demand. Consequently, the demand for electronic components has increased, and the current technical trend takes an orientation toward diversification of the functions, the increase of density, and high efficiencies.
Further, the application of the soft ferrite material for use in making electronic components such as electromagnetic impediment eliminating devices and power transmission devices has been finely divided based on characteristics, frequency bands and the like.
In the manufacturing method also, a departure from the conventional powder metallurgical method was observed, and stacked type components are briskly studied and have been put to practical uses. This method has settled in the technical field of the small chip components such as ceramic electronic components as a major part.
Generally, the soft ferrite material which is used in manufacturing small chip components such as chip inductors, chip beads, chip arrays, chip LC filters and chip trans requires a high inductance. This soft ferrite material includes the Mn--Zn ferrite, Ni ferrite, Ni--Zn ferrite and Ni--Cu--Zn ferrite.
The Mn--Zn ferrite has a high permeability, and shows a low power loss. Therefore, it is used as the magnetic core for power supply transformers power line filters and the like. However, the Mn--Zn ferrite shows a low radio frequency characteristic, and therefore, it cannot be applied to a frequency band of over 1 MHz. At present, the magnetic core materials which are usable in the frequency band of over 1 MHz are Ni ferrite, Ni--Zn ferrite, and Ni--Cu--Zn ferrite.
Meanwhile, the conventional manufacturing method for the soft ferrite material is carried out in the following manner. That is, a sintering process is conducted at a temperature of 1000.degree.-1400.degree. C. for 1-5 hours. However, the inner electrodes of the electronic components such as the chip inductor and the chip bead filter are made of silver (Ag). The above described sintering temperature exceeds the melting point of Ag (960.degree. C.). Therefore, not only is the sintering temperature too high, but also the manufactured components show much loss at the general frequency band of 500 KHz-20 MHz. The required inductance cannot be met. In order to lower the sintering temperature for the soft ferrite material, generally the particle size of the magnetic core material is finely crushed to 0.01-1 .mu.m. Thus the energy level of the particles is made to reach the ground state (metastable state), and the material moving space between the particles is made to be increased during sintering so as to promote low temperature sintering.
However, the manufacturing method resorting to fine crushing requires expensive facilities and a complicated process, with the result that the manufacturing cost is increased. Further, there is a problem in putting the process to practical use.
As another example, there is a method in which an ingredient such as B.sub.2 O.sub.3 is used (Japanese Patent Application Laid-open No. Sho-64-45771). There is still another method in which sintering is carried out by adding a flux such as ZnO or V.sub.2 O.sub.5 so as to induce a surface spread of the particles (Japanese Patent Application Laid-open No. Sho-60-210572).
However, in the method in which the low melting point compound is added, the behavior of the Co component for improving the frequency characteristics is impeded, thereby lowering the sintering effect. Further, in this method, the additives exist in the liquid phase at a temperature lower than the sintering temperature of the soft ferrite material as the matrix. Therefore, the additives are spread on the grain boundaries to promote sintering. Due to the segregation of the additives, an inductance drop and loss occurs. Further, they interact with the inner electrode made of Ag, or are spread on the Ag electrode. Consequently, the magnetic properties (inductance, Q-factor) of the chip inductor are degraded, with the result that product reliability deteriorates.